One of the most perplexing issues with regards to dengue virus infection is the finding that whereas primary infection with one serotype leads to long term protection against the same serotype, infection with a heterologous serotype leads to a wide spectrum of illness ranging from mild to severe and in some cases fatal. The mechanisms for this outcome are far from clear and provide for a major challenge to produce an effective dengue vaccine since the potential exists that the vaccine may in fact potentiate severe disease. Our lab has been studying the acute response to dengue virus in patients with dengue fever (DF) and dengue hemorrhagic fever (DHF) in attempts to delineate biological markers that can distinguish DF from DHF in an endemic population. During this process our lab has documented the following: a) early (day 1-3) during dengue infection, the plasma of these patients contain a number of cytokines and chemokines that we submit drive the mobilization of memory B cells which develop into plasma blasts and represent >50% of the B cells in the blood b) the plasma blasts are highly susceptible to apoptosis by factors in the late (day 7-10) sera of these patients and/or by their interaction with Fas-L expressing NK cells c) there is a marked increase in the frequencies of plamacytoid dendritic cells (pDC's) which express immature phenotype d) there is a muted memory B cell response against the heterologous dengue virus in these patients. Based on these preliminary findings, our lab has formulated a series of questions, which are aimed at defining in more detail the chain of immunological events that occur during acute dengue infection with the aim to define the events that are associated with disease severity. Thus, the objectives of this proposal are to carry out detailed studies of a) levels of the chemokines/cytokines that are present in the sera of dengue patients during acute infection and identify homing markers on the plasma blasts and correlate mild asymptomatic v/s severe disease b) attempt to define the mechanisms that lead to enhanced apoptosis of the plasma blasts c) examine the relationships between KIR/MHC polymorphisms that contribute to the quality of interactions between NK cell and monocytes and dendritic cells (DC's) and d) studies of the phenotypic and functional heterogeneity of monocytes/DC's that correlates with disease severity. We have assembled an outstanding team of immunologists, virologists and pediatric dengue clinicians with a long track record of working on dengue viral disease to address each of the specific aims outlined and submit that the results from these studies will not only contribute to the understanding of the pathogenesis of dengue infection but may also provide clues to effective vaccine formulation.